The present invention relates to a hot melt adhesive composition, to a method of filling voids with the adhesive composition and to articles filled or coated with the hot melt adhesive composition. More particularly, it relates to block copolymers containing glass fibers, aluminum powder and hollow inorganic silicate microspheres, to a method of filling voids with such compositions and to articles, filled or coated with the compositions.
Hot melt adhesives are well known in the prior art. These materials are conveniently applied to a substrate in the molten state and upon cooling form an adhesive bond. However, a deficiency common to most of the hot melt adhesives of the prior art is their tendency to soften and flow at elevated temperatures, as, for example, 70.degree. to 100.degree. C. with a resulting loss of bond strength. Consequently, these materials are not suitable for use over a broad temperature range.
Attempts to upgrade the softening and flow temperatures have involved using very high molecular weight resinous materials and/or crosslinking of the resin. These methods have resulted in materials with higher softening points and flow temperatures. However, in most cases the resulting material was not adapted to thermal processing because its higher molecular weight and/or crosslinked structure engendered extremely high application viscosity. Thus, these materials were not suitable for use as hot melt adhesives.
In the manufacture and repair of metal bodies of automobiles and appliances, solder compositions containing lead are frequently used to fill cavities and voids. These lead solders are extremely dense and can add a significant increment to the weight of the metal body. They present a health hazard which mandates special handling to protect workers engaged in the soldering and cavity filling operations. Curable adhesives such as epoxies are generally unsatisfactory for such cavity and void filling applications because they require careful metering of the components to provide good physical properties and bond strength, because they take too long to cure to a sandable state and because they have rather poor weather resistance. Conventional hot melt adhesives are also unsatisfactory for cavity and void filling applications because they cannot be sanded rapidly at assembly line speed, they do not readily accept paint, exhibiting "telegraphing" or "bleed-through", and they do not withstand the high temperatures necessary for the subsequent cure of paint overcoats. "Telegraphing" or "bleed-through" are terms used to describe the revelation of difference in composition of the substrate when it has been painted, because of a difference in reflectivity between the painted metal and the painted adhesive composition.
U.S. Pat. No. 3,650,999 discloses block copolymer comprising hard polyester segments and soft polyamide segments having improved adhesion and high temperature performance obtained by reacting a crystalline polyester, a C.sub.18 to C.sub.54 polycarboxylic acid and a primary diamine. This poly(esteramide) in common with other hot melt adhesives has deficiencies in creep resistance at temperatures above 150.degree. C. in the range up to 205.degree. C. and above and in shrinkage when the hot melt is cooled to room temperature after application. These deficiencies have been overcome to a considerable degree by incorporating a metal powder into the block copolymer to yield a cavity filling composition which possesses good sandability and paint acceptance. However, the metal powder copolymer composition can lack adequate impact resistance especially at low temperatures and can sag excessively at elevated temperatures. Attempts to improve the impact resistance by introducing an energy-absorbing reinforcement were generally unsuccessful and added a further complication of blinding of the sanding disc, making sanding extremely difficult.